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Corrosion behaviour of TiB2 reinforced aluminium based in situ metal matrix composites
Accepted Manuscript Title: Corrosion behaviour of TiB2 Reinforced Aluminium based In-situ Metal Matrix Composites Author: G.S. Pradeep Kumar R. Keshavamurthy Prachi Kumari Chirag Dubey PII: DOI: Reference:
S2213-0209(16)30040-4 http://dx.doi.org/doi:10.1016/j.pisc.2016.04.025 PISC 188
To appear in: Received date: Accepted date:
13-1-2016 4-4-2016
Please cite this article as: Kumar, G.S.P., Keshavamurthy, R., Kumari, P., Dubey, C.,Corrosion behaviour of TiB2 Reinforced Aluminium based In-situ Metal Matrix Composites, Perspectives in Science (2016), http://dx.doi.org/10.1016/j.pisc.2016.04.025 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Corrosion behaviour of TiB2 Reinforced Aluminium based In-situ Metal Matrix Composites Pradeep Kumar G.S.1, R.Keshavamurthy2, Prachi Kumari1, Chirag Dubey1 1
Automobile Engineering, Dayananda Sagar College of Engg. , Bangalore, Karnataka, India Mechanical Engineering, Dayananda Sagar College of Egg. , Bangalore, Karnataka, India +91 9739466913, [email protected]
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Abstract This paper focuses on corrosion characteristics of cast and forged Aluminium 6061 based composites reinforced with TiB2 particles. Composites were synthesized by in-situ technique using Potassium Hexafluorotitanate salt (K2TiF6) and Potassium Tetrafluroborate (KBF4)
us
halide salts by stir casting route at a temperature of 850oC. Cast Aluminum alloy and its insitu composites were subjected to open die drop forging at a temperature of 500oC. Both cast
an
and forged alloy 6061 and in-situ composites were then subjected to microstructure studies, salt spray test. Salt spray test was conducted as per ASTM B117 standard test procedure using 5% Sodium Chloride test solution. Result reveals that, forged alloy and its in-situ
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composites exhibited improved corrosion resistance compared to cast ones. Keywords: Al6061, TiB2, salt spray test, forging.
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Introduction Aluminum 6000 series alloys are being used extensively for various engineering applications
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due to its superior strength to weight ratio. Possessing dimensional stability, excellent structural rigidity and a low thermal expansion coefficient [1], its excellent formability allows
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easier usage for secondary processes such as hot extrusion, rolling and forging [2, 3]. The properties of pure Aluminium can be improved to a great extent by using reinforcements in the form of ceramic such as SiO2, Al2O3 and SiC as particulates to name the more common ones [4,5]. Aluminium alloys are used in the automobile industry for production of pistons, connecting rod etc. [6, 7] Research now also extends to Aluminium composites that use continuous/discontinuous fibers, whiskers with varying volume fractions. Compared with the commonly used ceramic reinforcements, TiB2 takes the higher stand due to its exceptional characteristics in terms of higher thermodynamic stability, higher modulus (530*10^3 GPa), higher hardness and low density [8,9].
In recent years, researchers are focusing on
development of alumibum-TiB2 in-situ metal matrix composites owing to its several advantages over ex-situ technique. The exothermic reaction and better wettability of TiB2 with Aluminium is causative for improved in situ preparation of Aluminium alloys allowing a better interface bonding and less differences between thermo physical properties during
Page 1 of 6
heating between the two[9, 10]. Poor wettability of ceramics leads to uneven dispersion, lower mechanical properties and high porosity [6]. Selection of fabrication method is important as the mechanical properties of the resulting alloy depend on it [3, 6]. Amongst various in situ fabrication methods utilizing liquid metallurgy, the more prevalent method is
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to use halide salts. On the other hand, it is reported that secondary process such as extrusion, forging rolling are commonly used methods to processing aluminum alloys for many engineering applications. Further, secondary processing of aluminum based composites offers
cr
several benefits like uniform dispersion of reinforcements, elimination of casting defects, excellent union between matrix and reinforced phase etc. Among all secondary process exist;
us
forging method induces a significant improvement in terms of strength and stiffness of aluminum components. Literature review reveals that limited work been carried out in terms
an
of characterizing corrosion behavior of cast and forged Aluminum-TiB2 composite processed by in-situ reaction technique. In light of above, the present work focuses on development of cast and hot forged Al6061-TiB2 in-situ metal matrix composites and characterizing its
M
microstructure and corrosion behavior by using salt spray test.
Experimentation Al6061-TiB2 composites were prepared by first melting the base Aluminium 6061, obtained
d
in a graphite crucible using electric resistance furnace. Molten Aluminium alloy maintained
te
at a temperature of 8600C was added with halide salts, Potassium Hexafluorotitanate salt (K2TiF6) and Potassium Tetrafluroborate (KBF4) in a stoichiometric ratio to obtain, 5wt%
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TiB2, 10wt% TiB2. The cast alloy and its composites were subjected to open die hot forging. More details on the composite preparation and forging are available in our earlier works [7]. Cast and forged samples of alloy and its composites were machined to the size of 10*10*10 mm cubes and polished metallographically. The polished samples were then subjected to salt spray test as per ASTM B 117 standard test procedures, using salt spray chamber (MakeCalture Instruments, Model-NSSOI-01). The test samples were suspended in salts spray chamber at 300 from the vertical, containing dissolved 5% NaCl (AR Grade) in distilled water for a period of total of 96 hours and weight loss was recorded at an interval of every 12 hours. The pH and the temperature of the solution were maintained at 7.08 and 350C respectively. After the test, the samples were cleaned with running water and air dried before measuring for weight loss. RESULTS AND DISCUSSION Optical Microstructure
Page 2 of 6
Fig.1 (a-b) shows optical micrographs of cast and forged Al6061-alloy and Al6061-TiB2 Insitu composites. It is seen from the micrographs that TiB2 particles are dispersed in aluminum matrix in a fairly uniform manner under both cast and forged conditions. However, when compared with the cast composites, forged composites shows more uniformity in distribution
b
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a
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of TiB2 particles, which can be attributed to thermo-mechanical deformation during forging.
(a) Cast Al6061-10wt%TiB2 (b) Forged Al6061--10wt%TiB2 Fig.1 (a-b) Optical micrographs of Al6061-TiB2 In-situ composites
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Salt Spray corrosion Test
Fig.2 shows variation of weight loss in salt sprayed specimens of cast and forged Al6061
d
alloy and its composites. It is observed from the graph that weight loss increases with increase in test duration for all the specimens. It is also observed that, composites exhibited
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marginally lower weight loss compared with alloys in cast and forged conditions.
Fig. 2 Salt spray test results for cast and forged alloy and its composites Corrosion resistance of the in situ metal matrix composites increases with increase in TiB2 content in cast and forged conditions. On the other hand, forged composites shows least weight loss compared to cast ones which may be attributed to the fact that, during thermochemical processing, TiB2 particles are distributed more homogeneously compared with agglomerated cast composite which is evident from micro-structure studies. Further, the
Page 3 of 6
improved corrosion behaviour of forged composites over cast ones may also be attributed to reason that some of the micro-porosities and other defects observed in casting process are minimized in forged composites. It is noticed from the graph that weight loss increases rapidly up to 36 hours of exposure to salt spray action. After first 36 hours of exposure,
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oxidation of aluminium leads to a formation of a protective oxide layer on the surface of the specimens. This oxide layer protects the underlying surface from further corrosion attack. However, the thickness and continuity of oxide/protection layer largely depends on the
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distribution and percentage of TiB2 content in both cast and forged composites. Decrease in the rate of weight loss after 36 hours of salt spray exposure may also be attributed to
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formation of oxide layer which contributes to weight gain. Further, it is also observed from the graph that the rate of oxidation is lower for forged samples when compared with cast
an
ones. The cast alloy and its composites allow a larger amount of the base metal to be oxidized. TiB2 reinforced samples are protected to a larger extent, owing to the fact that the quality of passive oxide layer is better than the non-reinforced samples.
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Corrosion mechanism in In-situ composite
Fig.3 (a-b) shows scanning electron micrographs of salt sprayed In-situ composites. It can be
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clearly observed that micrograph; In-situ composite is largely affected by pitting corrosion. This phenomenon is influenced by TiB2 content and distribution. Among various corrosion
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mechanisms proposed by researchers, Mechanism of breakdown of passive layer by chloride ions and penetration of Cl through the oxide layer is the significant mechanism observed.
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Formation of a protective passive layer on the surface exposed after initial with the solution/attacking Cl ions during initial exposure.
a
Cast Al6061-10wt%TiB2
bb
Forged Al6061--10wt%TiB2
Fig 3 (a-b) SEM micrographs of Cast and forged Al6061-TiB2 In-situ composites. Localised dissolution at the metal/oxide interfacing producing a localized acidic environment increases the matrix dissolution beneath the oxide layer to form blister. The surface area which is unable to form the passive layer becomes instrumental in the formation of pits [11-
Page 4 of 6
12].Formation of blister which is purported to be under oxide layer through the adsorption of Cl ions is favoured by positively charged surface of the samples with the intermetallic partners acting as local cathodes to the Cl ions. These blisters under favourable conditions enlarge and rupture to form a new open pit [13].
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Conclusion: Al6061-TiB2 In-situ composites were synthesized by liquid metallurgy route and hot forged successfully. Corrosion of cast and forged alloy and its composites increases with increase in
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salt spray duration. Decrease in weight loss was observed with increased content of TiB2 in both cast and forged conditions. However, under all conditions studied the corrosion
us
resistance and forged alloy and its composites exhibited lower weight loss compared its cast counter parts.
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M
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Bibliography [1] Kumar, S., Chakraborty, M., Sarma, V.S. and Murty, B.S., 2008. Tensile and wear behaviour of in situ Al–7Si/TiB2 particulate composites. Wear, 265(1), pp.134-142. [2] Ramesh, C.S., Pramod, S. and Keshavamurthy, R., 2011. A study on microstructure and mechanical properties of Al 6061–TiB2 in-situ composites. Materials Science and Engineering: A, 528(12), pp.4125-4132. [3] Ramesh, C.S. and Safiulla, M., 2007. Wear behavior of hot extruded Al6061 based composites. Wear, 263(1), pp.629-635. [4] Zou, X.G., Miyahara, H., Yamamoto, K. and Ogi, K., 2003. Quantitative evaluation on wear resistance of aluminum alloy composites densely packed with SiC particles. Materials science and technology, 19(11), pp.1527-1530. [5] Birol, Y., 2007. Response to thermal exposure of the mechanically alloyed Al–Ti/C powders. Journal of materials science, 42(13), pp.5123-5128. [6] Bharath, V., Nagaral, M., Auradi, V. and Kori, S.A., 2014. Preparation of 6061Al-Al2O3 MMC's by Stir Casting and Evaluation of Mechanical and Wear Properties. Procedia Materials Science, 6, pp.1658-1667. [7] Pradeep Kumar, G.S., Keshavamurthy, R., Ramesh, C.S. and Bharathesh, T.P., 2015. Mechanical properties of Hot forged Al6061-TiB2 In-situ Metal Matrix Composites. Materials Today: Proceedings, 2(4), pp.3107-3115. [8] Gao, Q., Wu, S., Lü, S., Duan, X., & Zhong, Z. (2015). Preparation of in-situ TiB2 and Mg2Si hybrid particulates reinforced Al-matrix composites. Journal of Alloys and Compounds, 651, 521-527. [9] Tee, K., Lu, L., & Lai, M. O. (1999). In situ processing of Al–TiB2 composite by the stircasting technique. Journal of Materials Processing Technology, 89, 513-519. [10] Smagorinski, M. E., Tsantrizos, P. G., Grenier, S., Cavasin, A., Brzezinski, T., & Kim, G. (1998). The properties and microstructure of Al-based composites reinforced with ceramic particles. Materials Science and Engineering: A, 244(1), 86-90. [11] Frankel, G.S., 1998. Pitting corrosion of metals a review of the critical factors. Journal of the Electrochemical Society, 145(6), pp.2186-2198. [12] McCafferty, E., 2003. Sequence of steps in the pitting of aluminum by chloride ions. Corrosion Science, 45(7), pp.1421-1438.
Page 5 of 6
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M
an
us
cr
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[13] Natishan, P.M. and O’grady, W.E., 2014. Chloride ion interactions with oxide-covered aluminum leading to pitting corrosion: a review. Journal of The Electrochemical Society, 161(9), pp.C421-C432.
Page 6 of 6
S2213-0209(16)30040-4 http://dx.doi.org/doi:10.1016/j.pisc.2016.04.025 PISC 188
To appear in: Received date: Accepted date:
13-1-2016 4-4-2016
Please cite this article as: Kumar, G.S.P., Keshavamurthy, R., Kumari, P., Dubey, C.,Corrosion behaviour of TiB2 Reinforced Aluminium based In-situ Metal Matrix Composites, Perspectives in Science (2016), http://dx.doi.org/10.1016/j.pisc.2016.04.025 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Corrosion behaviour of TiB2 Reinforced Aluminium based In-situ Metal Matrix Composites Pradeep Kumar G.S.1, R.Keshavamurthy2, Prachi Kumari1, Chirag Dubey1 1
Automobile Engineering, Dayananda Sagar College of Engg. , Bangalore, Karnataka, India Mechanical Engineering, Dayananda Sagar College of Egg. , Bangalore, Karnataka, India +91 9739466913, [email protected]
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2
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Abstract This paper focuses on corrosion characteristics of cast and forged Aluminium 6061 based composites reinforced with TiB2 particles. Composites were synthesized by in-situ technique using Potassium Hexafluorotitanate salt (K2TiF6) and Potassium Tetrafluroborate (KBF4)
us
halide salts by stir casting route at a temperature of 850oC. Cast Aluminum alloy and its insitu composites were subjected to open die drop forging at a temperature of 500oC. Both cast
an
and forged alloy 6061 and in-situ composites were then subjected to microstructure studies, salt spray test. Salt spray test was conducted as per ASTM B117 standard test procedure using 5% Sodium Chloride test solution. Result reveals that, forged alloy and its in-situ
M
composites exhibited improved corrosion resistance compared to cast ones. Keywords: Al6061, TiB2, salt spray test, forging.
d
Introduction Aluminum 6000 series alloys are being used extensively for various engineering applications
te
due to its superior strength to weight ratio. Possessing dimensional stability, excellent structural rigidity and a low thermal expansion coefficient [1], its excellent formability allows
Ac ce p
easier usage for secondary processes such as hot extrusion, rolling and forging [2, 3]. The properties of pure Aluminium can be improved to a great extent by using reinforcements in the form of ceramic such as SiO2, Al2O3 and SiC as particulates to name the more common ones [4,5]. Aluminium alloys are used in the automobile industry for production of pistons, connecting rod etc. [6, 7] Research now also extends to Aluminium composites that use continuous/discontinuous fibers, whiskers with varying volume fractions. Compared with the commonly used ceramic reinforcements, TiB2 takes the higher stand due to its exceptional characteristics in terms of higher thermodynamic stability, higher modulus (530*10^3 GPa), higher hardness and low density [8,9].
In recent years, researchers are focusing on
development of alumibum-TiB2 in-situ metal matrix composites owing to its several advantages over ex-situ technique. The exothermic reaction and better wettability of TiB2 with Aluminium is causative for improved in situ preparation of Aluminium alloys allowing a better interface bonding and less differences between thermo physical properties during
Page 1 of 6
heating between the two[9, 10]. Poor wettability of ceramics leads to uneven dispersion, lower mechanical properties and high porosity [6]. Selection of fabrication method is important as the mechanical properties of the resulting alloy depend on it [3, 6]. Amongst various in situ fabrication methods utilizing liquid metallurgy, the more prevalent method is
ip t
to use halide salts. On the other hand, it is reported that secondary process such as extrusion, forging rolling are commonly used methods to processing aluminum alloys for many engineering applications. Further, secondary processing of aluminum based composites offers
cr
several benefits like uniform dispersion of reinforcements, elimination of casting defects, excellent union between matrix and reinforced phase etc. Among all secondary process exist;
us
forging method induces a significant improvement in terms of strength and stiffness of aluminum components. Literature review reveals that limited work been carried out in terms
an
of characterizing corrosion behavior of cast and forged Aluminum-TiB2 composite processed by in-situ reaction technique. In light of above, the present work focuses on development of cast and hot forged Al6061-TiB2 in-situ metal matrix composites and characterizing its
M
microstructure and corrosion behavior by using salt spray test.
Experimentation Al6061-TiB2 composites were prepared by first melting the base Aluminium 6061, obtained
d
in a graphite crucible using electric resistance furnace. Molten Aluminium alloy maintained
te
at a temperature of 8600C was added with halide salts, Potassium Hexafluorotitanate salt (K2TiF6) and Potassium Tetrafluroborate (KBF4) in a stoichiometric ratio to obtain, 5wt%
Ac ce p
TiB2, 10wt% TiB2. The cast alloy and its composites were subjected to open die hot forging. More details on the composite preparation and forging are available in our earlier works [7]. Cast and forged samples of alloy and its composites were machined to the size of 10*10*10 mm cubes and polished metallographically. The polished samples were then subjected to salt spray test as per ASTM B 117 standard test procedures, using salt spray chamber (MakeCalture Instruments, Model-NSSOI-01). The test samples were suspended in salts spray chamber at 300 from the vertical, containing dissolved 5% NaCl (AR Grade) in distilled water for a period of total of 96 hours and weight loss was recorded at an interval of every 12 hours. The pH and the temperature of the solution were maintained at 7.08 and 350C respectively. After the test, the samples were cleaned with running water and air dried before measuring for weight loss. RESULTS AND DISCUSSION Optical Microstructure
Page 2 of 6
Fig.1 (a-b) shows optical micrographs of cast and forged Al6061-alloy and Al6061-TiB2 Insitu composites. It is seen from the micrographs that TiB2 particles are dispersed in aluminum matrix in a fairly uniform manner under both cast and forged conditions. However, when compared with the cast composites, forged composites shows more uniformity in distribution
b
an
us
cr
a
ip t
of TiB2 particles, which can be attributed to thermo-mechanical deformation during forging.
(a) Cast Al6061-10wt%TiB2 (b) Forged Al6061--10wt%TiB2 Fig.1 (a-b) Optical micrographs of Al6061-TiB2 In-situ composites
M
Salt Spray corrosion Test
Fig.2 shows variation of weight loss in salt sprayed specimens of cast and forged Al6061
d
alloy and its composites. It is observed from the graph that weight loss increases with increase in test duration for all the specimens. It is also observed that, composites exhibited
Ac ce p
te
marginally lower weight loss compared with alloys in cast and forged conditions.
Fig. 2 Salt spray test results for cast and forged alloy and its composites Corrosion resistance of the in situ metal matrix composites increases with increase in TiB2 content in cast and forged conditions. On the other hand, forged composites shows least weight loss compared to cast ones which may be attributed to the fact that, during thermochemical processing, TiB2 particles are distributed more homogeneously compared with agglomerated cast composite which is evident from micro-structure studies. Further, the
Page 3 of 6
improved corrosion behaviour of forged composites over cast ones may also be attributed to reason that some of the micro-porosities and other defects observed in casting process are minimized in forged composites. It is noticed from the graph that weight loss increases rapidly up to 36 hours of exposure to salt spray action. After first 36 hours of exposure,
ip t
oxidation of aluminium leads to a formation of a protective oxide layer on the surface of the specimens. This oxide layer protects the underlying surface from further corrosion attack. However, the thickness and continuity of oxide/protection layer largely depends on the
cr
distribution and percentage of TiB2 content in both cast and forged composites. Decrease in the rate of weight loss after 36 hours of salt spray exposure may also be attributed to
us
formation of oxide layer which contributes to weight gain. Further, it is also observed from the graph that the rate of oxidation is lower for forged samples when compared with cast
an
ones. The cast alloy and its composites allow a larger amount of the base metal to be oxidized. TiB2 reinforced samples are protected to a larger extent, owing to the fact that the quality of passive oxide layer is better than the non-reinforced samples.
M
Corrosion mechanism in In-situ composite
Fig.3 (a-b) shows scanning electron micrographs of salt sprayed In-situ composites. It can be
d
clearly observed that micrograph; In-situ composite is largely affected by pitting corrosion. This phenomenon is influenced by TiB2 content and distribution. Among various corrosion
te
mechanisms proposed by researchers, Mechanism of breakdown of passive layer by chloride ions and penetration of Cl through the oxide layer is the significant mechanism observed.
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Formation of a protective passive layer on the surface exposed after initial with the solution/attacking Cl ions during initial exposure.
a
Cast Al6061-10wt%TiB2
bb
Forged Al6061--10wt%TiB2
Fig 3 (a-b) SEM micrographs of Cast and forged Al6061-TiB2 In-situ composites. Localised dissolution at the metal/oxide interfacing producing a localized acidic environment increases the matrix dissolution beneath the oxide layer to form blister. The surface area which is unable to form the passive layer becomes instrumental in the formation of pits [11-
Page 4 of 6
12].Formation of blister which is purported to be under oxide layer through the adsorption of Cl ions is favoured by positively charged surface of the samples with the intermetallic partners acting as local cathodes to the Cl ions. These blisters under favourable conditions enlarge and rupture to form a new open pit [13].
ip t
Conclusion: Al6061-TiB2 In-situ composites were synthesized by liquid metallurgy route and hot forged successfully. Corrosion of cast and forged alloy and its composites increases with increase in
cr
salt spray duration. Decrease in weight loss was observed with increased content of TiB2 in both cast and forged conditions. However, under all conditions studied the corrosion
us
resistance and forged alloy and its composites exhibited lower weight loss compared its cast counter parts.
Ac ce p
te
d
M
an
Bibliography [1] Kumar, S., Chakraborty, M., Sarma, V.S. and Murty, B.S., 2008. Tensile and wear behaviour of in situ Al–7Si/TiB2 particulate composites. Wear, 265(1), pp.134-142. [2] Ramesh, C.S., Pramod, S. and Keshavamurthy, R., 2011. A study on microstructure and mechanical properties of Al 6061–TiB2 in-situ composites. Materials Science and Engineering: A, 528(12), pp.4125-4132. [3] Ramesh, C.S. and Safiulla, M., 2007. Wear behavior of hot extruded Al6061 based composites. Wear, 263(1), pp.629-635. [4] Zou, X.G., Miyahara, H., Yamamoto, K. and Ogi, K., 2003. Quantitative evaluation on wear resistance of aluminum alloy composites densely packed with SiC particles. Materials science and technology, 19(11), pp.1527-1530. [5] Birol, Y., 2007. Response to thermal exposure of the mechanically alloyed Al–Ti/C powders. Journal of materials science, 42(13), pp.5123-5128. [6] Bharath, V., Nagaral, M., Auradi, V. and Kori, S.A., 2014. Preparation of 6061Al-Al2O3 MMC's by Stir Casting and Evaluation of Mechanical and Wear Properties. Procedia Materials Science, 6, pp.1658-1667. [7] Pradeep Kumar, G.S., Keshavamurthy, R., Ramesh, C.S. and Bharathesh, T.P., 2015. Mechanical properties of Hot forged Al6061-TiB2 In-situ Metal Matrix Composites. Materials Today: Proceedings, 2(4), pp.3107-3115. [8] Gao, Q., Wu, S., Lü, S., Duan, X., & Zhong, Z. (2015). Preparation of in-situ TiB2 and Mg2Si hybrid particulates reinforced Al-matrix composites. Journal of Alloys and Compounds, 651, 521-527. [9] Tee, K., Lu, L., & Lai, M. O. (1999). In situ processing of Al–TiB2 composite by the stircasting technique. Journal of Materials Processing Technology, 89, 513-519. [10] Smagorinski, M. E., Tsantrizos, P. G., Grenier, S., Cavasin, A., Brzezinski, T., & Kim, G. (1998). The properties and microstructure of Al-based composites reinforced with ceramic particles. Materials Science and Engineering: A, 244(1), 86-90. [11] Frankel, G.S., 1998. Pitting corrosion of metals a review of the critical factors. Journal of the Electrochemical Society, 145(6), pp.2186-2198. [12] McCafferty, E., 2003. Sequence of steps in the pitting of aluminum by chloride ions. Corrosion Science, 45(7), pp.1421-1438.
Page 5 of 6
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te
d
M
an
us
cr
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[13] Natishan, P.M. and O’grady, W.E., 2014. Chloride ion interactions with oxide-covered aluminum leading to pitting corrosion: a review. Journal of The Electrochemical Society, 161(9), pp.C421-C432.
Page 6 of 6